Until recently, a variety of hardware manufacturing and assembly enterprises, such as the boat manufacturing industry, have made many of the parts employed in their products as partially finished components—those which are finished on only one side (the side visible to and engaged by the user). As market demands have now driven many of these companies to produce many, if not all, of their parts as completely finished components, these manufacturers have turned to the use of resin transfer molding (RTM) processes of the type which have been well established in the automotive and aircraft industry. However, because of the relatively low volume per part, these enterprises cannot afford the very substantial cost of very robust (typically steel) molds used in RTM processes employed in the automotive and aircraft industries. Instead, the RTM systems that have been used for low margin, limited part production, such as in the marine manufacturing industry, are typically made from less robust materials, such as epoxy resins, which suffer from poor part yield and are prone to frequent failures.
A standard RTM process typically places a fiber preform of the part to be molded within a mold cavity, whose geometric shape is defined by the volumetric spacing between an outer, female mold half, and an inner male mold half. The two mold halves or elements are joined together and effectively sealed at their circumferential edges, capturing a perimeter portion of a structural (fiber) preform therebetween. Non-limiting examples of fibers used in fiber preforms include fiberglass, graphite, carbon and Kevlar, and the fibers are often braided or woven into a sheet form.
In order to mold a part, it is customary practice to inject a liquid resin (such as an epoxy resin, polyester resin, and the like) into the mold cavity, typically by a negative pressure created by means of a vacuum pump. The liquid resin is usually heated to mold temperature, i.e., catalyzed, in a heated pressure tank and pressure-injected through a resin flow line coupled to the mold cavity. The mold cavity has one or more bleed vents that are ported to a collection reservoir to which the vacuum pump is coupled. Transfer/injection of the liquid resin into the mold cavity may be facilitated by introducing pressurized gas into the pressure tank, which in turn causes the pressurized resin to flow into the mold cavity. As the pressurized, heated resin is introduced into the mold cavity, it thoroughly wicks or is absorbed by the fiber preform.
At various times after pressure-injecting the heated resin into the mold cavity, the mold operator will open the line coupled between the collection reservoir and the mold cavity, in order to bleed resin from the mold cavity and into the collection reservoir. The collection reservoir may include a sight glass to enable the operator to view released resin, and determine, based upon on his experience and skill, whether it contains air bubbles. If air bubbles are visible, indicating that the cavity has not been completely filled with resin, the operator closes the line to the collection reservoir, and continues to introduce additional pressurized resin into the mold cavity. This resin bleeding and inspection process is repeated, as necessary, until the operator is satisfied that the mold cavity has been completely purged of air, and filled with resin (which has saturated the fiber preform).
Each repetition of the resin bleeding and inspection step is time consuming, primarily due to the fact that, after the operator bleeds resin into the collection reservoir, the pressure tank must be pressurized again. Typically, this bleeding and inspection step is repeated at least three or four times for each molded article, making the resin transfer molding process a relatively lengthy and expensive task. Resin is also wasted each time the operator is required to bleed the resin into the collection reservoir to inspect it for air bubbles. Because this occurs several times during the molding of each article, the volume of wasted resin and associated cost can be substantial.
Another drawback of such a conventional molding process is the fact that it relies upon operator judgment to hopefully correctly determine (based upon what the operator perceives is a lack of air bubbles in the resin bled into the collection reservoir) if and when the mold cavity and fiber preform have been completely filled with resin. Even if the operator accurately observes no air bubbles in resin that has been bled into the collection reservoir, this is no assurance that the mold cavity is free of voids. The operator can only rely on what the bleed port or ports reveal; he has no picture of the entirety of the mold. This inability of the operator to ensure that the mold is void-free results in poor repeatability, and lower overall quality and yield of the articles being produced. It also increases glass print transfer into the finished surface of the part.
In addition, irrespective of the skill and experience of the operator, the architecture and manner of assembly of the two mold halve themselves can result in a less than satisfactory product. The circumferential edge regions at which the male and female mold halves are joined together customarily capture a perimeter portion of a fiber preform placed between the two mold halves. As this can cause bunching together or even pulling of the fibers of the preform, it leads to inconsistencies in the thickness and density of the preform material at the joined mold edges and also within the mold cavity. This not only causes variations in the dimensions of the molded article, but can create variations in the flow of resin through the mold cavity, leading to air pockets, that are not discovered until the part has been cured and removed from the mold.